**Core Concept**
The patient's presentation is suggestive of a hydrocele, which is a fluid accumulation in the tunica vaginalis surrounding the testis. The key feature of a hydrocele is its transilluminant nature, meaning that light can pass through it.

**Why the Correct Answer is Right**
Given the patient's presentation of a cystic, transilluminant, painless scrotal swelling with separate testes palpable, the diagnosis of a hydrocele is likely. The procedure of choice for this condition is a **surgical exploration** or **hydrocelectomy**, which involves making an incision in the scrotum to drain the fluid and repair any underlying defects. This procedure is usually performed under local anesthesia.

**Why Each Wrong Option is Incorrect**
**Option A:** A herniotomy is a surgical procedure used to repair a hernia, which is not the primary diagnosis in this case.
**Option B:** A testicular biopsy is a procedure used to diagnose testicular cancer or other conditions affecting the testis, but it is not indicated in this case where the testes are palpable and separate.
**Option C:** An orchiectomy is a surgical procedure that involves removing the testis, which is not indicated in this case where the testes are palpable and separate.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hydroceles can be congenital or acquired, and they can be associated with underlying conditions such as testicular cancer or infection.

**Correct Answer:** D.